Each cylinder of an internal combustion engine is equipped with one or more gas exchange valves (e.g., intake and exhaust valves) that are cyclically opened during normal operation. In a conventional engine, the valves are opened by way of a camshaft/rocker arm arrangement. The camshaft includes one or more lobes oriented at particular angles corresponding to desired lift timings and amounts of the associated valves. The cam lobes are connected to stem ends of the associated valves by way of the rocker arm and associated linkage components. As the camshaft rotates, the cam lobes come into contact with a first pivoting end of the rocker arm, thereby forcing a second pivoting end of the rocker arm against the stem ends of the valves. This pivoting motion causes the valves to lift or open against a spring bias. As the cam lobes rotate away from the rocker arm, the valves are released and allowed to return to their closed positions.
When a cylinder is equipped with more than one of the same type of gas exchange valve (e.g., more than one intake valve and/or more than one exhaust valve), all valves of the same type are typically opened at about the same time. And in order to reduce a number of camshafts, cam lobes, and/or rocker arms required to open the multiple valves, a valve bridge is often used to interconnect the same type of valves with a common rocker arm.
A valve bridge is generally T-shaped, having arms that extend between the stem ends of two like valves. The second end of the rocker arm engages a center portion of the valve bridge, between the arms. With this configuration, a single pivoting motion imparted to the center of the valve bridge by the rocker arm results in lifting of the paired valves by about the same amount and at about the same timing. A lash adjuster can be associated with the ends of each valve stem in the valve bridge and used to remove clearance that exists between the valves and corresponding seats (and/or between other valve train components) when the valve is released by the rocker arm. The lash adjuster helps to ensure sealing of the cylinder during the ensuing combustion process.
An exemplary valve actuation assembly is disclosed in U.S. Pat. No. 4,924,821 that issued to Teerman on May 15, 1990 (“the '821 patent”). The '821 patent discloses a system having hydraulic lash adjusters disposed within a valve bridge to engage stem ends of two associated exhaust valves. A screw actuator extends from a rocker arm into a socket recessed within a center of the valve bridge. Engine oil is fed through a gallery in the rocker arm, through the screw actuator, and through the valve bridge to the hydraulic lash adjusters.
Although the valve actuation assembly of the '821 patent may be suitable for many applications, it may still be less than optimal. For example, the engine oil is only fed to the lash adjusters as long as the associated engine is operational. During engine restart and before the oil can be pressurized and directed to the valve actuation assembly, there may not be enough oil in the valve actuation assembly to ensure proper operation of the assembly.
The valve actuation system of the present disclosure is directed towards overcoming one or more of the problems set forth above and/or other problems of the prior art.